The effects of harsh radiation environments, such as are encountered in space, military, or nuclear power applications, are known to have deleterious effects on semiconductor devices and circuits. Recent studies have further shown that prolonged exposures to even low levels of radiation can degrade the performance of electronic devices. Compounding these effects, the continued reduction in feature size of modern commercial integrated circuits (ICs) can result in these devices becoming even more sensitive to transient ionizing radiation effects. Ionizing radiation produces electron-hole pairs (i.e., electrical charge) in semiconductor material that may be collected by potentially sensitive circuit nodes. The generation and collection of this unintended electrical charge within the circuit can alter the performance and reliability of specific circuit structures within the device.
It is particularly desirable to be able to measure and monitor changes in the electrical charge collection within a device to detect changes in its radiation response with age. Age-related changes in the radiation response of numerous types of semiconductor devices have been reported by several groups in recent years. See A. P. Karmarkar et al., “Aging and Baking Effects on the Radiation Hardness of MOS Capacitors,” IEEE Trans. Nucl. Sci. 48(6), 2158 (2001); and V. S. Pershenkov et al., “Effect of Aging on Radiation Response of Bipolar Transistors,” IEEE Trans. Nucl. Sci. 48(6), 2164 (2001). Studies of aging in ICs suggest that achieving a detailed understanding of some of the natural changes that occur in these devices over long storage times requires examination of the electrical and material properties of specific age-affected circuit structures within a die.
It is also important that the measurements be of comparable spatial resolution to the circuit structures comprising the device, and that the measurements be non-destructive to permit re-measurement of the same device at a later time to determine if aging effects have occurred in the device's charge collection response. Historically, the greatest obstacle to monitoring the radiation response of electronic devices used in long-lived electrical systems has been the destructive nature of most traditional radiation testing techniques. The accumulation of total dose damage with each subsequent re-test of a device can change its radiation response, (e.g. dose-rate, single event upset, single event latch-up, burn-out) independent of any other hidden aging effects. Moreover, existing radiation test techniques typically do not provide sufficient spatial information about the exact location of the radiation response changes within a die to permit targeted application of modern failure analysis techniques, such as are applied during the production phase of ICs. In particular, many of the high-sensitivity trace element analysis techniques most applicable to the analysis of aging effects (e.g., time-of-flight secondary ion mass spectroscopy and focused ion beam x-ray spectroscopy) can analyze regions only microns in diameter. Therefore, to enable the practical use of these targeted analytical techniques requires that specific circuit structures exhibiting aging effects be identified and located with micron-scale spatial resolution. Unfortunately, precise positional information can seldom by deduced from conventional broad-beam x-ray or electron beam radiation testing, or from broad-beam laser testing. Furthermore, conventional, non-destructive laser systems which employ beam-defining apertures to control the exposure area of the laser beam cannot produce usable beam spots much below tenths of millimeters in diameter. Particularly as IC circuit density increases and feature sizes decrease, apertured laser testing does not provide sufficiently detailed spatial information to directly identify individual age-affected circuit structures (e.g., diffused resistors, diodes, transistors and their component gates, drains and sources) with sufficient spatial accuracy to efficiently target state-of-the-art failure analysis techniques.
In most modern semiconductor devices, it is only the charge produced in the top few tens of microns of silicon that can be collected by the electrically active regions of the device; electron-hole pairs produced deeper in the silicon (i.e., in the heavily doped silicon beneath the epitaxial layer) recombine before reaching sensitive circuit nodes. Thus, while it is true that the exposure of a silicon device to severe gamma- or x-ray fluxes gives rise to electron-hole pair production throughout the entire silicon die (and package and circuit board and the entire system), the vast majority of this electrical charge has no path to directly affect the internal operation of the IC. If the charge is not produced within a diffusion length of the electrically active regions of the circuit, it will merely recombine, or be trapped, without perturbing the circuit.
A focused, milliwatt-watt infrared laser can produce as many electron-hole pairs in the electrically active regions of a silicon IC as are produced during severe radiation exposures of MeV gamma-rays or keV x-rays. The apparently huge difference in power between the laser and gamma- or x-ray exposure conditions is reconciled not when viewed in terms of how much energy the device is exposed to, but rather how much of that energy is actually deposited in the silicon device where it can affect the device's electrical operation. When viewed in this context, very few of the incident gamma- or x-rays actually produce electron-hole pairs within the silicon and even fewer of these electron-hole pairs are produced within the collection volume of the electrically active regions of the device. Therefore, infrared laser-based irradiation testing can produce dose-rate response in semiconductor devices that replicates the response to much more penetrating ionizing radiations.
Furthermore, laser-based probing can detect aging effects without the damage associated with particle beam or x-ray exposure techniques. Thus, dose-rate failure testing of ICs can be performed repeatedly, non-destructively, and inexpensively on a bench top. It is important to note that broad beams of energetic x-rays or electrons provide higher fidelity simulations of actual high-energy radiation environments than the infrared laser. Indeed, such exposures reproduce many bulk material radiation effects (e.g., shock, thermal effects, and secondary radiations) that the laser cannot. However, in the context of photocurrent collection and circuit electrical operation, the additional energy deposition deep in the sample only produces electron-hole pairs that may contribute to thermal and mechanical effects, but not to electrical behavior.
Among the most widely-used commercial lasers for laser-based radiation testing are Nd:YAG (Nd:YVO4) lasers which operate at a wavelength of approximately 1064 nanometers with a corresponding penetration depth of approximately 300 microns in silicon. The laser-based irradiation apparatus of the present invention preferably uses 904-nm-wavelength laser light from a diode laser, having a penetration depth, or absorption length, of about 30 microns. As a consequence of this difference in penetration depth, the Nd:YAG laser requires much more power to achieve the same dose rate-equivalent charge density in the electrically active regions of the IC. This is because the electrically active regions of the IC are typically in the top 15 microns of the die. Thus, the 904-nm-wavelength laser light can achieve the same density of charge in the electrically active regions of the silicon (e.g., the top 15 microns) at a lower power than the Nd:YAG laser because the shorter wavelength light produces electrical charge only in the regions of the device from which it can be collected. This, in turn, allows the duty cycle of the laser's operation to be greater and allows for hundreds of thousands of laser shots to be performed during the collection of a scanned, micron-resolution image of the device's charge (i.e. photocurrent) collection. Additionally, the diode laser can be fired on-demand, rather than at a fixed repetition rate, which also facilitates its use in a scanned, data-collection mode. Finally, laser diodes that emit at other wavelengths, including 1064 nm, can used to create varying depths of penetration of the laser light and thereby enable the investigation of the depth dependence of a device's charge collection efficiency.
In a first embodiment of the present invention, a scanned, focused, infrared laser dose-rate equivalent exposure apparatus has been developed to measure and image the photocurrent collection across an entire silicon die. Due to its low power consumption and low heat generation, combined with its high duty cycle, ability to be focused without undue lens heating, and on-demand triggering, the focused laser irradiation apparatus permits the micron-scale imaging of the photocurrent collection (i.e., dose-rate response) across an entire IC die in a fully automated scan sequence. Comparisons of these dose-rate response images from before, during, and after accelerated aging of a device, or from periodic sampling of devices from fielded operational systems (such as avionic, nuclear power, or radiation facility electronics) allows precise identification of those specific age-affected circuit structures within a device that merit further quantitative analysis with targeted materials or electrical testing techniques. The high spatial resolution inherent in the focused laser irradiation apparatus directly indicates where such quantitative analysis techniques can be applied. By identifying the underlying physical cause of the device malfunction, it is then possible to determine the time-dependence of the overall effect, and predict the end-state of the dose-rate response in the aged device.
A second embodiment of the laser-based irradiation apparatus comprises a broad-beam, dose rate-equivalent exposure apparatus. Unlike the focused laser irradiation apparatus, which can identify specific circuit structures within the die that have changed with age, the broad-beam laser irradiation apparatus can determine if aging has affected the device's overall functionality. Additionally, for devices exposed to dynamically changing radiation environments, the broad-beam laser irradiation apparatus can replicate the time varying, dose-rate intensity of a space or hostile military environment. This embodiment can be combined with the synchronized introduction of external electrical transients into a device under test (DUT) to simulate the electrical effects of the surrounding circuitry's response to a radiation exposure.